Hitachi SH7032 Hardware Manual page 230

External-Pin Round-Robin Mode: External-pin round-robin mode switches the priority levels of
channel 0 and channel 1, which are the channels that can receive transfer requests from external
pins DREQ0 and DREQ1. The priority levels are changed after each (byte or word) transfer on
channel 0 or channel 1 is completed. The channel which just finished the transfer rotates to the
bottom of the priority order. The priority levels of channels 2 and 3 do not change. The initial
priority order after a reset is channel 3 > channel 2 > channel 1 > channel 0.
Figure 9.5 shows how the priority order changes when channel 0 and channel 1 transfers are
requested simultaneously and a channel 0 transfer is requested again after both channels finish
their transfers. The DMAC operates as follows:
1. Transfer requests are generated simultaneously for channels 1 and 0.
2. Channel 1 has a higher priority, so the channel 1 transfer begins first (channel 0 waits for
transfer).
3. When the channel 1 transfer ends, channel 1 becomes the lowest priority.
4. The channel 0 transfer begins.
5. When the channel 0 transfer ends, channel 0 becomes the lowest priority.
6. A channel 0 transfer request occurs again.
7. The channel 0 transfer begins.
8. When the channel 0 transfer ends, the priority order does not change, because channel 0 is
already the lowest priority.
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